In the manufacture of semiconductor packaging products, such as, substrates, conductive metal pattern on a substrate is defined by conductive paste deposition through a contact mask.
A method for screening conductive paste onto ceramic green sheet is described in U.S. Pat. Nos. 4,068,994 and 4,362,486.
Screening masks used for the production of high circuit density devices have closely spaced fine dimension etched patterns. Paste screening through such masks leaves paste residue entrapped in the fine mask features. The entrapped residue requires that the mask be cleaned after one or more screening passes to maintain conductive pattern integrity in subsequent screening and provide defect-free product.
A method for in-line mask cleaning after each screening pass is described in U.S. Pat. No. 4,304,536, in which the masks are sprayed with a solvent to remove paste and subsequently dried.
In the fabrication of multi-layer ceramic substrates of the type described in U.S. Pat. No. 4,245,273, conductive pastes selected for screening are based on a variety of polymer binder-solvent vehicle systems, selection of which is dictated by the requirement for a particular circuit pattern and compatibility of the paste with greensheet materials.
Various commonly used paste types in the manufacture of multi-layer ceramic substrates are described in U.S. Pat. No. 4,245,273.
An automated in-line mask cleaning process using perchloroethylene as a cleaning solvent for Mo metal masks used in multi-layer-ceramic technology is described in U.S. Pat. Nos. 4,383,040 and 4,362,486.
Halogenated hydrocarbon solvents such as Perchloroethylene, 1,1,1-Trichloroethane, 1,1,2-trichloroethylene, methylene chloride, CFC-113, and CFC-112 traditionally have been used for cleaning/degreasing purposes in many industrial and consumer product applications. Perchloroethylene is a commonly used solvent in chemical, pharmaceutical and microelectronics industry, and is widely used in dryclean industry.
Chlorinated hydrocarbon solvents, particularly perchloroethylene and 1,1,1-trichloroethane are also used in multi-layer ceramic manufacturing. These solvents are used in cleaning processes for masks and accompanying equipment which are used in screening metal-polymer conductive paste onto ceramic green sheet for delineation of wiring and via metallurgy pattern.
Preference for chlorinated solvents in industrial cleaning processes is due to the fact that these solvents are non-flammable (no flash point) and are highly effective cleaning solvents in degreasing and removal of organic and inorganic-organic composite residues. Perchloroethylene is an especially suitable solvent for high efficiency cleaning in manufacturing environment as described in U.S. Pat. No. 4,383,040, since it is fast acting, fast drying due to relatively high vapor pressure, leaves no residue after drying, and can be recycled.
However, a major problem with halogenated hydrocarbon solvents such as perchloroethylene and trichloroethane, is that these have been found to have harmful effects on human health and the environment, therefore their use in industrial processes has become highly restricted in recent years. Specifically, this category of solvents have been identified as Hazardous Air Pollutants (HAP-solvents) which are on the OSHA list of Suspected Carcinogens (cancer causing agents), and these are among the SARA Title-III reportable (Superfund Amendment & Re-Authorization Act) compounds that are on the TRI (Toxic Release Inventory) chemicals list. In addition to these environmental and health hazards, 1,1,1-Trichloroethane is also an Ozone Depleting Substance (ODS) and thus has already been phased out while 1,1,2-trichloroethylene has been targeted for phase out by the year 2002.
With the currently imposed strict environmental regulations on hazardous/toxic chemicals, use of perchloroethylene, trichloroethane, and related chemicals in industrial applications has become very costly since it requires regular permit renewal from the state, strict compliance with rules requiring monitoring, record keeping, and precise data reporting of toxic air emissions. Adding to the cost are maintenance of emission control devices, hazardous waste disposal, and the necessary controls to assure worker safety. There is also a problem of toxic/hazardous waste and the high cost of disposal due to the presence of residual chlorinated solvent.
In-line mask cleaning processes described in U.S. Pat. Nos. 4,383,040 and 4,362,486 use perchloroethylene for automatic cleaning, i.e., they are designed for real time operation to provide cycle time compatibility with high throughput screening. This requires use of the cleaning solvent in large volume to provide for effective cleaning within the cycle time required for other steps in the screening process. Use of perchloroethylene in large volumes in turn results in high level of toxic solvent emissions and generation of large volume of hazardous solid and liquid waste. The solid waste which mainly comprises metals and inorganic components with small amount of polymer binder-solvent in the paste, also contains residual chlorinated solvent and thus is disposed-off as hazardous waste at twice the cost of a waste having non-chlorinated solvent contamination and at ten times the cost of a non-hazardous waste having no organic solvent contamination. Another problem with the use of perchloroethylene in mask cleaning is that the washed-off paste residue settles as a hard cake at the bottom of the cleaning tank as the solvent partially evaporates, which makes it difficult to recover the precious metals from the solids.
Because of the environmental concerns and the health hazards associated in the use of chlorinated solvents for industrial cleaning processes in general, there is currently a major focus on identifying environmentally safe replacements. Various chemical suppliers and cleaning equipment manufacturers have made available several alternate organic solvents that are relatively safe and mostly exempt from environmental regulations, as well as water-based cleaning solutions and the necessary equipment for alternate organic solvent and water-based cleaning.
In manufacturing applications, aqueous cleaning is generally preferred over solvent cleaning because of the flammability, cost, and waste disposal issues associated with the use of organic solvents.
Therefore, this invention is concerned with mask cleaning and particularly replacement of chlorinated solvents with a more environmentally desirable method of cleaning masks, extrusion heads, and other accompanying equipment that is used in screening conductive paste onto greensheet in the manufacture of multi-layer ceramic. Furthermore, the invention is especially concerned with cleaning of screening masks with water-based cleaning solutions based on a combination of surfactants, and alkaline detergent compositions based on alkali metal salts, alkali metal hydroxides, and combination thereof, as non-hazardous replacement of perchloroethylene, 1,1,1-trichloroethane, trichloroethylene, and related chlorinated solvents.
The inventors have discovered a group of aqueous cleaning compositions that can be used for cleaning paste from the screening masks. These aqueous cleaning compositions can be derived from: (a) combination of non-ionic and ionic surfactants in water; (b) alkaline detergent compositions based on alkali metal salts as silicate/metasilicate, carbonate, tribasic sodium phosphate, sodium tripolyphosphate, and combinations thereof; (c) highly alkaline solutions based on alkali metal salts, alkali metal hydroxides, and mixture thereof with alkanolamines.
Unlike chlorinated solvents, the aqueous cleaning compositions according to this invention have no problem of toxic air emissions or health hazards, minimizes waste, and allows recovery of precious metals from the removed paste residue in the wash liquid and thus provides multiple benefits.